1. Field of the Invention
The present invention relates to a single-sided paper phenolic resin copper-clad laminate for use in a printed wiring board.
2. Description of the Related Art
As shown in FIG. 4, a single-sided paper phenolic resin copper-clad laminate (hereinafter referred to as xe2x80x9csingle-sided laminatexe2x80x9d for short) 41 of prior art is composed of a phenolic resin impregnated paper base (hereinafter referred to as xe2x80x9cpaper basexe2x80x9d) 42 formed by laminating a predetermined number of paper phenolic resins and having copper foils 43 laminated onto a face side of the paper base 42 and resists 44 formed of a synthetic resin such as an epoxy resin and applied to prescribed areas on the face side of the paper base 42.
The single-sided laminate 41 as described above has been increasingly demanded in recent years because of its excellence in electrical and physical characteristics and workability in application for a printed wiring board and also its inexpensiveness.
Lead solder consisting of 63% of tin (Sn) and 37% of lead (Pb) has been used in the past for mounting electronic components on a printed wiring board. To be precise, the eutectic composition thereof consists of 61.9% of Sn and 38.1% of Pb (all the percentages are by mass), but the solder consisting of 63% of Sn and 37% of Pb and having a melting point of 183xc2x0 C. is usually regarded as eutectic solder.
When electronic components 45 are mounted using this lead solder on the face side of the single-sided laminate 41 shown in FIG. 4, cream solder (lead solder) is applied to the surface of the copper foils 43 laminated on the face side of the paper base 42 using a printer (not shown), then the electronic components 45 are placed by a mounting machine (not shown).
Next, the single-sided laminate 41 having the electronic components 45 mounted thereon is placed on a belt conveyor 52 at an inlet IN of a reflow furnace 51 using both hot air and infrared radiation (shown in FIG. 5), and conveyed into the reflow furnace 51 at a constant speed. During conveyance, the cream solder is melted by infrared radiant heat from a panel heater 53 and hot air from a blower 54 in the reflow furnace 51, and the copper foils 43 of the single-sided laminate 41 and terminals of the electronic components 45 are reflow-soldered to each other, thereby forming solder joints 46. The single-sided laminate 41 which has gone through the reflow furnace 51 is taken out at an outlet OUT of the reflow furnace 51, and the mounting of the electronic components 45 is completed.
FIG. 6 is a temperature profile for evaluation from the inlet IN to the outlet OUT of the reflow furnace 51, and as shown in this figure, the peak temperature Tm in the reflow furnace 51 is about 220xc2x0 C., which is higher by about 40xc2x0 C. than the melting point of lead solder (about 180xc2x0 C.).
The single-sided laminate 41 used for evaluation measures 92xc3x9773.3xc3x970.9 mm, and the electronic components 45 are small and do not require large heat capacity.
Under the conditions as described above, the single-sided laminate 41 warped 2.0 mm at most during the reflow process, and it was confirmed that a product having this single-sided laminate 41 mounted causes no problem with quality.
However, in an extremely rare case where large heat capacity is required depending on the size or material of the electronic components 45, the peak temperature Tm in the reflow furnace 51 exceeds 220xc2x0 C., and the single-sided laminate 41 can warp more than 2.0 mm.
In this case, when the single-sided laminate 41 is screwed onto a product, the single-sided laminate 41 and the solder joints 46 are easily subject to cracking. And, when the peak temperature Tm in the reflow furnace 51 exceeds 245xc2x0 C., the single-sided laminate 41 undergoes discoloration (scorches) on the reverse side.
Recently, electronic equipments such as OA equipment, communication devices and the like have been developed remarkably. For these electronic equipments, printed wiring boards, on which a lot of electronic components are connected with lead solder, are used. Similar printed wiring boards are employed for many circuits, for example, the control units also in home electric appliances, industrial production machines and so on.
However, electronic equipments, electric appliances and the like (hereafter referred to as xe2x80x9celectronic equipmentsxe2x80x9d all together) once produced are to be scrapped. Although scrapped electronic equipments are partly recycled for reuse, most of them are crushed and often buried in the ground without recycling.
Since lead solder contains nearly 40% of lead (Pb) as described above and is toxic to human bodies, animals, plants and the like, it must be duly discarded. Pb in electronic equipments (printed wiring boards) buried in the ground gradually dissolves in acid water such as acid rain, and pollutes soil and groundwater so as to be taken into animals and plants, and further human bodies.
In order to eliminate the pollution against the natural environment and the human bodies, lead-free solder has been developed in recent years. Examples of lead-free solder include Snxe2x80x94Agxe2x80x94Cu types, Snxe2x80x94Ag types, Snxe2x80x94Cu types, Snxe2x80x94Sb types, Snxe2x80x94Zn types and the like.
When any type of these lead-free solders is used in place of lead solder, the heat capacity provided for the single-sided laminate 41 needs to be increased for the reflow process in which the electronic components 45 are mounted on the single-sided laminate 41. This is because the melting point of the lead-free solders is around 210 to 227xc2x0 C. except some (Snxe2x80x94Zn types having a low melting point) and higher by approximately 40 to 50xc2x0 C. than the melting point 183xc2x0 C. of the conventional lead solder.
Accordingly, the peak temperature Tm in the reflow furnace 51 must be raised to 250 to 260xc2x0 C., which is about 30 to 40xc2x0 C. higher than conventionally. The single-sided laminate 41, which does not warp so much as to cause critical problems in the reflow process using the conventional lead solder, warps more than 2.0 mm under the above conditions. Therefore, a product with such a single-sided laminate 41 may have its quality deteriorated, and the improvement in this point has been required.
The present invention has been made in view of the above needs, and has an object to provide a single-sided paper phenolic resin copper-clad laminate (single-sided laminate) that warps only slightly even when the peak temperature in a reflow furnace is raised to a degree adapted for the melting point of lead-free solder (for example, 255xc2x0 C.) to enable the reflow process in which lead-free solder is used to mount electronic components.
In order to achieve the above-mentioned object, according to a first aspect of the present invention, in a single-sided laminate composed of a phenolic resin impregnated paper base (paper base) having copper foils laminated on and resists applied on the face side thereof, resists formed of the same material as the resists on the face side are applied on the reverse side of the paper base.
According to a second aspect of the present invention, in the first aspect of the present invention, the resists applied on the reverse side of the paper base are matched with the resists applied on the face side in position and configuration.